Trees in the line of Loma Fire

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The underbrush explodes in flames near Loma Prieta Way in the Santa Cruz Mountains near Los Gatos on Sept. 27. (Gary Reyes -- Bay Area News Group)

The remains of vegetation that burned on Loma Chiquita Road are evident as firefighters work to control the Loma Fire in the Santa Cruz Mountains, 11 miles west of Morgan Hill on Sept. 29. (Patrick Tehan -- Bay Area News Group)

A Cal Fire firefighter walks along Summit Road in Santa Clara County on Sept. 28. Firefighters worked to contain the wildfire in the Santa Cruz Mountains that has resulted in evacuations and threatened homes in the area. (Dan Honda -- Bay Area News Group)

SANTA CRUZ MOUNTAINS >> The Santa Cruz Mountains are home to giant coast redwoods, Douglas firs, knobcone pines and tanbark oaks, along with other native trees. There is also the occasional whiff from the many non-native eucalyptus.

Come fiery conditions, though, such as the recent Loma Fire, which burnt through 4,474 acres for more than 17 days, those majestic stands turn into threatening fuel sources and some trees burn more easily than others.

“In general, species differ in their contribution to fire behavior,” said J. Morgan Varner, a fire ecologist in the Seattle office of the U.S. Forest Service. Many people first think about trees with high oil content when they guess which trees burn first, “but what generally determines flammability are the not-as-interesting cast leaves on the forest floors,” Varner said.

The Loma Fire spread was based on fuel and slope, burning primarily through leaf litter, dry chaparral, knobcone pines and gray pines, said Edgar Orre, forester at Cal Fire.

Most surface fires, or fires that stay low in the forest, burn through leaves on the ground. Varner said some native oaks and pines are notable for their flammability because they shed a lot, and have longer leaves and needles that create a “fluffy” covering on the forest floor. With space between the layers, it’s a high-oxygen environment, making it very combustible, so fire moves across the landscape easily.

Faster spreading fires, or crown fires, occur when individual trees ignite to the very top. Species such as Douglas firs that don’t naturally prune and retain their lower branches are often prone to torching, Varner said. That occurs when flames leap upward using lower branches as rungs.

“The Loma Fire crowned in short stretches,” said Orre.

Flames leapt up on knobcone pines, oaks, Douglas firs and also some redwoods, he said.

Wildfires scorch forests but don’t necessarily kill every tree on their path. The Santa Cruz Mountains are more fire-prone than some other parts of the Santa Cruz County, said Orre. As a result these mountains have big populations of fire-adapted trees.

Cones of knobcone pines, for example, remain closed for years until a fire opens them up, allowing them to release seeds.

“Knobcone pines burned quite readily in the Loma Fire,” Orre said. Now as the embers cool down, knobcones will “start the process of healing all over again,” he said.

Most of the native oaks and older Douglas firs also survived the fire well, according to Orre.

Other trees that also burn well are those with oil content such as conifers, pines, bay laurels and eucalyptus, said Gregory Gilbert, senior forest ecologist at UC Santa Cruz. Eucalyptus trees tend to be especially flammable due to their fragrant volatile oils that ignite at very low temperatures.

However, eucalyptus were not a contributing factor in the Loma Fire, Orre said.